RE: OTV 3 post-manouevre elements and a small mystery

From: Ted Molczan via Seesat-l <>
Date: Sun, 10 Aug 2014 12:17:16 -0400
The following TLE is based on Kevin Fetter's and my observations of Aug 9-10 UTC:

OTV 3                                                    337 X 364 km
1 39025U 12071A   14222.05124835  .00000000  00000-0  00000-0 0    02
2 39025  43.5156 164.2522 0020000  99.4865 260.8427 15.73524265    09
Arc 20140809.05-0810.07 WRMS resid 0.033 totl 0.025 xtrk

Based on this solution, I can confidently state that the s/c spent some time in a significantly lower orbit sometime
between early Aug 4 UTC and early Aug 9 UTC. Below are its pre-manoeuvre elements: 

1 39025U 12071A   14216.09424805  .00000000  00000-0  00000-0 0    09
2 39025  43.5256 200.3762 0019515  34.3091 325.1355 15.72763563    06

As has been noted previously, observers found the object running more than 18 min. early on Aug 9, five days after it
was last seen. When I observed it in its present orbit, on Aug 09 at 02:47 UTC, it was about 1115 s early relative the
pre-manoeuvre orbit. The present orbit has nearly the same orbital period as the pre-manoeuvre orbit; therefore, it
cannot account for the 1115 s early arrival, and there must have been another orbit change in the 5 day interim that
could account for early arrival. 

There are two possibilities. The first is that it manoeuvred to a lower orbit, which would have allowed it to move ahead
of where it would have been in the pre-manoeuvre orbit. Alternatively, it could have gone higher, which would have
caused it to fall behind the pre-manoeuvre orbit, and given sufficient time, could have caused it more than lap it in
reverse, so that it appeared to be early when we observed it on the 9th.

The RAAN of the present orbit enables us to determine whether it went lower or higher. A lower orbit would have caused
the RAAN to precess westward faster; a higher orbit was have precessed more slowly. My analysis rules out a higher
orbit, because the RAAN would have been about 1.4 deg greater than that of the present orbit, well outside the
uncertainty in the orbital elements. 

Time spent in a variety of lower orbits could account for the 1115 s early arrival, and in agreement with the present
RAAN to within about 0.2 deg. The present elements could be off by 0.2 deg, but not by 1.4 deg, so the higher orbit can
be confidently dismissed.

Below are some combinations of orbital altitude and dwell time in that orbit that could account for the early arrival:

Alt   n0    Period  Dwell
Km   rev/d    s       d
286  15.96   5407   0.887
302  15.90   5427   1.203
316  15.85   5445   1.710
330  15.80   5462   2.957
336  15.78   5469   4.174

For comparison, here are the corresponding values of the pre and post-manoeuvre orbit, derived from the above elements

351  15.73   5487   pre-manoeuvre
349  15.74   5484   post-manoeuvre

I do not know the purpose of the manoeuvre. I speculate that it could have been intended to adjust the ground track,
perhaps to optimize observation of some target on the Earth, or in preparation for landing in the not too distant
future. For either purpose, I suspect that the combination of a smaller manoeuvre and longer dwell time would have been
preferred, to conserve propellants. If there was some urgency, then lower orbit might have been preferred.

Ted Molczan

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Received on Sun Aug 10 2014 - 11:18:14 UTC

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